Here are a few key insights and habits these great writers shared:
She has a secret for knowing when to stop researching: "When the facts begin to repeat themselves, that's when I know it's time to begin writing." (Sometimes research can be a sophisticated form of procrastination.)
Deborah Heiligman: Almost all of her sources are primary. She read Darwin's autobiography and a two-volume set of letters from Emma's family. A line in one of Emma's letters made her cry. "I knew I was hooked. I got absolutely passionately attached to both Charles and Emma."